Spectral conversion has been widely applied in the field of luminescence and is potentially beneficial in photovoltaics. Among different approaches, down conversion shows a theoretical potential to enhance the limit of energy conversion efficiency of a single-junction solar cell from about 32% to 40%. For CIGS solar cells, the main current loss in the short wavelength region (<550 nm) comes from absorption of the CdS buffer layer with a relative small bandgap. As the spectral response of CIGS has an optimal efficiency in the longer wavelength (550-1000 nm) region, spectral down converter that converts photons with wavelength shorter than 550 nm to longer wavelength at a high luminescent efficiency has the potential to reduce the above current loss for CIGS solar cells. In this work, transparent ceramics of Y3Al5O12: Ce3+have been fabricated and applied as the down convertor for CIGS solar cells. Their absorption and photoluminescence properties have been investigated. The device conversion efficiencies of CIGS solar cells with and without the down converter have been measured and compared. The results show that optimized Y3Al5O12: Ce3+ceramics may bring about 9% enhancement to the current density, which enhanced the efficiency of CIGS solar cells from 17.3% to 18.9%. Compared with a pure antireflection coating with an enhancement of ~6%, the great enhancement in down-converter device demonstrates that the Y3Al5O12: Ce3+ceramics has a potential to further improve the performance of CIGS solar cells.